1. Field of the Invention
The present invention relates to an apparatus and method for increasing channel capacity of a mobile communication system. More particularly, the present invention relates to an apparatus and method using a hybrid phase shift keying (HPSK) modulation system.
2. Background of the Related Art
A code division multiple access (CDMA) is a method for detecting an original signal sent after spreading/modulating the signal using a specific code, and demodulating the spread/modulated signal by using an identical code as the code used in a sending side. A pseudo noise code composed of random numbers or a walsh code can be used as the specific code.
The walsh code is a kind of orthogonal code having almost no mutual relationship among codes, and since a communication channel is allotted to each unit, identical frequency resources can be allotted to many channels. That is, the channels of the walsh code can be easily classified by orthogonality, and the code can be applied to a phase shift keying (PSK), such as a quadrature PSK (QPSK) or a hybrid PSK (HPSK).
FIG. 1 shows a general walsh code is composed of a plurality of sets. As shown in FIG. 1, the walsh code of a first set is composed of “1”, and the walsh code of a second set is composed of two walsh codes of {1, 1} of W0 and {1, −1} of W1. In addition, the walsh code of a fourth set includes four walsh codes composed of {1, 1, 1, 1} of W0, {1, −1, 1, −1} of W1, {1, 1, −1, −1} of W2 and {1, −1, −1, 1} of W3, and the walsh code of an eighth set includes eight walsh codes composed of {1, 1, 1, 1, 1, 1, 1, 1} of W0, {1, −1, 1, −1, 1, −1, 1, −1} of W1, {1, 1, −1, −1, 1, 1, −1, −1} of W2, {1, −1, −1, 1, 1, −1, −1, 1} of W3, {1, 1, 1, 1, −1, −1, −1, −1} of W4, {1, −1, 1, −1, −1, 1, −1, 1} of W5, {1, 1, −1, −1, −1, −1, 1, 1} of W6 and {1, −1, −1, 1, −1, 1, 1, −1} of W7. That is, the walsh codes can be increased as 2n sets according to necessity of a user.
FIGS. 2A, 2B and 2C are views illustrating a transition status of the QPSK and HPSK spread signals. FIG. 2A is a view showing a phase transition status of the QPSK spread signal, FIG. 2B is a view showing a phase transition status of the HPSK spread signal and FIG. 2C is a view showing the size change of signals generated in each phase transition.
As shown in FIGS. 2A and 2B, in the QPSK spread, a peak power value increases at a 0° phase transition and phase changes by 180° phase transition are frequently generated. In the HPSK spread, a peak power value by the 0° phase transition is relatively lower than in the QPSK spread and phase changes by 180° phase transition are less frequently generated.
As shown in FIG. 2C, in the 0° phase transition, overshoot is generated at the peak power and the peak power value is increased, and in the 180° phase transition, a severe change of the power is generated. When the peak power is high, a power amplifier with a high output power must be designed, and since heat is generated in the system in proportion to the high output power, a cooling device is needed, thus increasing the size. Since frequency of generation of the 0° or 180° phase transition can be reduced by using the HPSK spread rather than using the QPSK spread, design of the amplifier can be eased by having smaller overshoot of the power generated 0° phase transition or smaller change of the power generated in case of the 180° phase transition.
FIG. 3 is a view showing an apparatus for increasing channel capacity of a mobile communication system in accordance with the conventional art. The apparatus is a device for spreading a reverse walsh code in a 3rd generation (3G) system which uses the HPSK. The 3G system is a mobile communication system for providing communication services of a motion picture level, such as wideband-CDMA (W-CDMA) or CDMA-2000.
As shown in FIG. 3, the apparatus includes first and second walsh code units 11 and 12 for generating a plurality walsh codes, a first adder 21 for adding a specific walsh code inputted from the first walsh code unit 11 and a signal inputted through a reverse pilot channel (R-pilot CH.) or a dedicated physical data channel (DPDCH), and a second adder 22 for adding a signal of another data channel and a signal inputted from the first walsh code unit 11. The apparatus also includes first and second gain units 31 and 32 for outputting output signals of the first and second adder 21 and 22 by respectively gain-amplifying, a first sum unit 41 for summing the output signals of the first and second gain units 31 and 32 and outputting the result, a third adder 23 for adding a specific walsh code inputted from the second walsh code unit 12 and a signal inputted through a reverse fundamental channel (R-FCH) or a dedicated physical control channel (DPCCH), a fourth adder 24 for adding a signal of another channel and a signal inputted from the second walsh code unit 12, third and fourth gain units 33 and 34 for outputting output signals of the third and fourth adder 23 and 24 by respectively gain-amplifying, a second sum unit 42 for summing the output signals of the third and fourth gain units 33 and 34. The apparatus also includes a walsh rotator 50 for outputting a complex scrambling signal for CDMA modulation, a first multiplier 61 for multiplying an I-complex scrambling signal Is outputted from the walsh rotator 50 and an output signal of the first sum unit 41, a second multiplier 62 for multiplying an I-complex scrambling signal Is outputted from the walsh rotator 50 and an output signal of the second sum unit 42, a third multiplier 63 for multiplying a Q-complex scrambling signal Qs and an output signal of the second sum unit 42, a fourth multiplier 64 for multiplying a Q-complex scrambling signal Qs outputted form the walsh rotator 50 and an output signal of the first sum unit 41, a third sum unit 43 for summing signals outputted from the first and third multipliers 61 and 63 and outputting the result as an I-signal, and a fourth sum unit 44 for summing signals outputted from the second and fourth multipliers 62 and 64 and outputting the result as a Q-signal.
The dedicated physical data channel and the dedicated physical control channel are used in the W-CDMA, and the dedicated data channel transmits voice or data and the dedicated control channel transmits control information. Also, the reverse pilot channel and the reverse fundamental channel are used in the CDMA-2000, the mobile communication terminal transmits the reverse pilot channel for allowing performance of synchronization detection of a base station, and the reverse fundamental channel transmits respective voice and high-speed data.
The operation of the conventional apparatus for increasing channel capacity of a mobile communication system will be briefly described as follows. The information inputted through the respective channels is spread in the first to fourth adders 21 to 24 by the walsh code outputted from the first and second walsh code units 11 and 12 or an orthogonal variable spreading function (OVSF). For the output signal of the first to fourth adders 21 to 24, a predetermined gain is obtained in the first to fourth gain units 31 to 34 then summed in the first and second sum units 41 and 42 through the I or Q channel path and outputted. After performing complex scrambling of combining the signals outputted through the first and second sum units 41 and 42 with the complex scrambling signal outputted from the walsh rotator 50, the signals are outputted as the I-signal and the Q-signal and then filtered.
FIG. 4 is a view illustrating general concept of the complex scrambling. The complex scrambling is performed by operating a vector multiple of a complex data signal (Ichip+jQchip) and a complex scrambling signal (Is+jQs), and it can be formed as Formula 1.
                                                                        I                +                                  j                  ⁢                                                                          ⁢                  Q                                            =                                                (                                                                                    I                        chip                                            *                                              I                        s                                                              -                                                                  Q                        chip                                            *                                              Q                        s                                                                              )                                +                                  j                  ⁡                                      (                                                                                            I                          chip                                                *                                                  Q                          s                                                                    +                                                                        Q                          chip                                                *                                                  I                          s                                                                                      )                                                                                                                          =                                                (                                                            I                      chip                                        +                                          j                      ⁢                                                                                          ⁢                                              Q                        chip                                                                              )                                *                                  (                                                            I                      s                                        *                    j                    ⁢                                                                                  ⁢                                          Q                      s                                                        )                                                                                                        =                                                A                  chip                                *                                  A                  s                                *                                  ⅇ                                      j                    ⁡                                          (                                                                        ϕ                          chip                                                +                                                  ϕ                          s                                                                    )                                                                                                                              (                  Formula          ⁢                                          ⁢          1                )            
The Achip and ejφchip are a size and an angle of the Ichip+jQchip signal, and the As and ejφs are a size and an angle of the Is+jQs signal. Therefore, the size of the I+jQ signal corresponds to the multiplied value of the size of the complex data signal and the complex scrambling signal, and the angle of the I+jQ signal corresponds to a sum of the angles of the complex data signal and the complex scrambling signal.
With reference to FIG. 4, when {1, −1} is inputted for the respective I-complex data signal and the Q-complex data signal, and {1, 1} and {1, −1} are inputted for the I-complex scrambling signal and the Q-complex scrambling signal, will be described as follows. Since the complex data signals are respectively {1, 1}, Ichip+jQchip coordinates values of {1, 1} are repeatedly generated, and the complex scrambling signals are displayed being divided into having respectively {1, 1} and {1, −1} values for the Is+jQs coordinates. When the complex data signal {1, 1} and the complex scrambling signal {1, 1} are multiplied, 45° of the complex data signal and 45° of the complex scrambling signal are added, thereby obtaining a signal having a phase of 90°, and when the complex data signal {1, 1} and the complex scrambling signal {1, −1} are multiplied, 45° of the complex data signal and −45° of the complex scrambling signal are added, thereby obtaining a signal having a phase of 0°. Therefore, the complex scrambling can convert a 0° phase transition generated when two consecutive identical data are inputted into 90° phase transition.
FIG. 5 is a view showing an embodiment of the conventional complex scrambling of the apparatus for increasing channel capacity in the mobile communication system. The HPSK using the eighth set walsh code will be descried with reference to FIG. 4.
The eighth set walsh code is a code which is used to spread an actual data (ID+jQD). When ID=1 and QD=−1 are inputted, a W0 code is used for an I channel path, a phase of a final modulation signal (I+jQ) is changed according to which walsh code is used for a Q channel path
When the complex scrambling is performed by using Wn=W1 in the Q channel path, all data of the final modulation signal obtain a phase of 0°, and accordingly, overshoot is generated in peak power at every data transition. However, when the complex scrambling is performed by using Wn=W1 in the Q channel path, the data of the final modulation signal has a phase of {0, −90, 90, 0, 0, −90, 90, 0}, and therefore, two times of zero crossing by 180° phase transition and one time of 0° phase transition are generated for data transition. That is, data transition using a walsh code of an even number sequence in WX can minimize 0° and 180° phase transitions than using a walsh code of an odd number sequence. Therefore, in the HPSK, the complex scrambling is performed by using walsh codes of an even number sequence when consecutive data have identical values. As shown in the above, in the apparatus for increasing channel capacity of the conventional mobile communication system, since an I/Q data has a high peak power when performing 0° or 180° phase transition, the efficiency of the power amplifier is decreased.
To solve the above problem, the apparatus for increasing channel capacity of the conventional mobile communication system which uses the HPSK communication method can not use all walsh codes, but an even sequence walsh code or an odd number sequence walsh code must be restrictively used. Therefore, since the conventional apparatus for increasing channel capacity in the mobile communication system must use a specific walsh code by controlling the number of the walsh code which can be used to send data by using a traffic channel, the transmission rate and capacity is decreased.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background